This invention relates to a supervisory system for monitoring a plurality of remote stations from a central station.
Large merchandise items, including agricultural equipment, industrial machinery and certain consumer products, are stored outdoors for display because it is costly to store such items inside an enclosure. Thus, such products are frequently stolen from dealer lots, shipping yards and farm yards. Alarm systems have been designed to protect such items, but such systems have not been used extensively because of their bulk, expense, ineffectiveness and/or unreliability.
For example, it is known to use remote supervisory circuits coupled to a central station via conductors, such as described in U.S. Pat. No. 3,588,890. However, as is typical of such systems, only a simple DC voltage source, such as a battery, is connected to the remote circuits. In this case, such a supervisory system can be defeated by connecting an equivalent impedance and/or potential source to the conductors and then cutting the connection to the remote circuit. It would be desirable to have an electronic supervisory system which cannot be so easily defeated.